2025ApJ...991..151B

We present a quantitative spectroscopic study of 13 blue supergiant stars in the Pinwheel Galaxy M101, based on data obtained with the Low Resolution Imaging Spectrometer available at the Keck I telescope. The average stellar metallicity decreases from ∼1.9 Z_⊙ near the center of the galaxy to ∼0.3 Z_⊙ at the optical outskirts. The galactocentric radial metallicity gradient is statistically consistent with previous studies of the gas-phase oxygen abundance from H II regions using the direct method. The H II region-based Cepheid metallicities used by A. G. Riess et al. in their determination of the Hubble constant H₀ are in substantial agreement with our measurements. The direct method gas-phase metallicities of the 18 star-forming galaxies we have analyzed so far, when adjusted upward for a mean ∼0.15 dex oxygen dust depletion factor, are in good agreement with those we infer from the supergiants, over a factor of 50 in metallicity. From the same data, we derive an expression for the metal-dependent depletion of oxygen in photoionized nebulae. Utilizing the flux-weighted gravity-luminosity relationship (FGLR) of blue supergiants, we measure a distance to M101, D = 6.5 ± 0.2 Mpc (μ = 29.06 ± 0.08), which is within 1σ from determinations based on the tip of the red giant branch and Cepheids. With M101 as a nearby Type Ia supernova host and using the observed standardized B-band magnitude of the supernova, our FGLR distance yields an independent value of H₀ = 72.5 ± 4.6 km s^–1 Mpc^–1.